Security elements are often used to indicate the authenticity and genuineness of various items, including banknotes, member cards, concert tickets, different kinds of commercial goods of value such as perfumes, fashion items or replacement parts for cars and planes, just to mention a few. The use of such security elements is becoming more and more widespread in view of increasing numbers of forged and bogus products, as the security element poses a difficulty to any counterfeiter in the reproduction of bogus products.
Security elements that are based on fluorescence phenomenon are widely used. Fluorescence is observed if a fluorescent material, typically a dye or pigment, is exposed to excitation radiation. It follows a decay of the excited material, causing emission as a wavelength different from the excitation wavelength. The response of the material (i.e. the intensity and/or color of the observed emission in response to the excitation) is indicative for the genuineness of the material protected by the security element. For instance, fluorescent materials are used for security elements present on banknotes, wherein an excitation of 348 nm wavelength causes a fluorescence emission in bright green yellow.
The level of security provided by such fluorescence-only security elements is moderate at the most, as many fluorescent materials are commercially available, and mainly prevents copying on a copying machine. A counterfeiter may thus easily come into possession of a suitable material for mimicking the behavior of an authentic security element. The emission spectra of commercially available markers (dyes or pigments) are well documented and furthermore can be measured by a counterfeiter that suspects them to be used to protect the authenticity of an article. Even if two or more dyes or pigments have been used in combination, e.g. for forming two patterns on a substrate, counterfeiters may purchase these and prepare a corresponding mixture thereof, in order to mimic the security mark of an article.
Further, conventional security elements are mainly produced by printing. The industrial printing technologies currently available for printing e.g. serialized codes include mostly inkjet (Continuous Ink Jet—CIJ or by drop on demand—DoD). The choice of suitable fluorescent products that can be incorporated in the ink formulation for these technologies is limited due to the small size of the printer head nozzles. Mostly soluble organic dyes or fine organic pigments are used to avoid nozzle clogging. These fluorescent products all have very short (nanosecond or shorter) emission lifetimes, which prevent from using the luminescence decay time as an authentication criterion with standard imaging devices such as hand-held devices.
Other authentication methods rely on the decay time of phosphorescent markers. For implementing such authentication method with a standard imager at relatively low frame rate (typically 50 frames per second), the decay time should be relatively long (tens to hundreds of milliseconds). The long afterglow phosphors currently available have coarse pigment particle sizes (>10 microns) which prevent them from being digitally printed. CIJ or DoD printers have tiny nozzles which would be clogged by the phosphor pigments and crushing the pigment particles to smaller sizes lead to a loss of their properties. When codes are printed such that they are not visible to unaided eye, there are even additional limitations on the choice of fluorescent products and more stringent requirements on ink formulation. In summary, current digitally printed secure fluorescent codes are restricted to the use of soluble organic dyes or fine pigments with short and non-specific fluorescence lifetime.
An additional problem with actual methods used for serialization of product with printed secure Dot-matrix codes on pre-printed labels is that the link between the label and the code is not secured. In other words, non-serialized pre-printed labels can be stolen by bad people who could potentially print their own serialization codes for counterfeiting products.
Further, such conventional security elements rely on the use of one species only for forming the security element. Nowadays, however, with the increasing trend of a product being manufactured at different places, there is a need for security elements that can provide a link between different places and different times, e.g. between pre-printed labels and a product code. This allows for providing a means for indicating the authenticity of a product along its chain of manufacture.
A more reliable authentication of luminescent marks with imaging devices can be achieved by exploiting the spectral properties of the emitted light, i.e. by analyzing the emission spectrum in the visible spectrum or in other spectral ranges, such as UV and IR. With a standard imaging sensor, performing multi (or hyper) spectral imaging, e.g. in the NIR (near infrared range) range, would require techniques such as: (1) custom Bayer-like filters (involving expensive developments) (2) Fabry-Perot configurations (currently bulky and fairly expensive), (3) complex cameras using AOTF (Acousto-Optic Tunable Filters, which also bulky and expensive), (4) switchable band-pass interference filters (with inconvenient moving parts), or (5) imaging spectrographs requiring push-brooms (unsuitable for handheld readers). Thus, the finer analysis of spectral properties generally requires complex, bulky and expensive equipment, and is difficult to implement in handheld devices or widely distributed authentication equipment.
Another means to achieve a more reliable authentication is a spectrometer. However, this device does not provide an image of the mark, making it unsuitable for code reading or geometrical checks of the printed mark.
Prior art authentication methods relying on imagers thus mainly rely on the type of emission observed. Yet, the intensity of the emission and/or the time-dependent emission behavior is generally not used for authentication purposes. Put differently, the prior art authentication methods generally check if the expected emitting species is present. Interactions between certain materials other than the emitting species, or interactions between different emitting species, are generally not used for authentication purposes. Also, the chemical surrounding of chemical species is typically not used for authentication purposes.
In addition, for Secure Track and Trace of products and/or for excise tax recovery enforcement of various goods, serialized digital marks (e.g. Dot-matrix or bar-codes) are printed on labels affixed to the products or printed directly onto the products or their packages. In order to prevent from copying or forging these codes, security inks are used. In some applications, fluorescent inks are used to print these codes which are detected and decoded using special readers which aim at measuring the ink luminescence properties for authentication. In some applications it is desired that the Public may also read and decode the security codes, which therefore have to be reflective in the visible spectral range (visible codes). This does not prevent from having secure codes that are visible and fluorescent.
Most of the security code readers are detecting ink luminescence over a relatively broad spectral range and hence are not taking advantage of the full specific luminescence properties (e.g. spectral shape of the emission). To guarantee a certain degree of covertness and further providing resistance to copy, the secured codes are optionally invisible to unaided eye. But in this case they cannot be decoded by the Public, which is a disadvantage for certain applications.
U.S. Pat. No. 7,079,230 B1 relates to authentication devices and methods and, more particularly, to portable hand-held device and a method for authenticating products or product packaging. In one embodiment of this patent document, a method of selecting a light-sensitive compound for application to a substrate and subsequent detection on the substrate is disclosed. The method includes irradiating the substrate with light, sensing an emission spectrum of the substrate in response to the irradiation, determining at least one peak wavelength of light within the emission spectrum, and selecting a light-sensitive compound that emits or absorbs light at a first wavelength in response to the irradiating light wherein the first wavelength is different from the at least one peak wavelength. In another embodiment, a method of authentication is described which includes producing an ink containing a first compound that emits light at a first discreet wavelength and a second compound that emits light at a second discreet wavelength, printing a readable image on a substrate with the ink, detecting a ratio of the first compound with the second compound on the substrate, indicating whether the ratio is within a range and reading the image. In one embodiment, one or more light-sensitive compounds, such as, for example, one or more fluorescent light-emissive compounds, is mixed with ink to be printed on a product or a product package. The system of this reference document requires the measurement of at least two different emission peaks and consequently requires a measuring device that contains two separate detectors, one for each emission peak.
WO 2013/050290 A1 describes a method for the automatic examination of the authenticity of value-indicating stamps and indicia comprising a luminescent area, the stamp or indicium being applied to the surface of a mail item. The surface of the item is irradiated with light of a wavelength of spectral range, a first image of the surface of the item is recorded by means of a camera system and said first image is evaluated with respect to the location of stamps or indicia applied thereto on the surface of the item. A comparison of evaluation of the image sections or image sections with stored luminescence patterns will lead, when these match, to a decision on the authenticity of every stamp or indicium.
While fluorescent dyes are widely used in authentication marks and security elements, the use of pigments, in particular phosphorescent pigments, is limited. Such phosphorescent pigments are often coarse materials that cannot be utilized in many printing applications, such as inkjet printing. Accordingly, their use is limited in practice.